Bruce Lee: The Art of Expressing the Human Body

If greater strength is desired, it will be necessary to make that muscle contract against a greater load until it has adapted to the overload. The essentials of training, therefore, are overload and adaptation.

The muscle strength has adapted specifically to your needs.

Excluding physical defects and some pathological conditions, your present physical condition is not static or fixed. Your physical condition merely reflects the specific adaptation of your body to your everyday life. In other words, you are trained for whatever activity you have been practicing, and no more.

Speed also depends upon strength . . . the stronger the individual, the faster he can run . . . moreover, endurance is based on strength.